I don't think we should switch to 120mm smoothbore for arjun mk3.This is because the original arde l52 gun created in early 90's had a muzzle velocity of 1650m/s.Since then a lot of technological improvement is done to it to make it more versatile,higher barrel life and higher barrel velocity.This is done as a result of partnership of arde and dmrl hence there is improved material(titanium and nickel cadmium alloys) in the inner barrel coating and better rifling techniques.The rifled arjun main gun can fire hesh round which is a vital requirement of the original army's psqr way back in 1985.The hesh round is higly effective for against enemy structures like hardened bunkers and concrete buildings as well as exposed enemy personnel.Also there is no technological barrier for arjun to switch to smoothbore if need arises since arde has already manufactured an indegeneous 125mm smothbore for licence production of t-90s bhisma tank(as russian has refused to provide tot for t-90s main gun and era).What we need in arjun mk2 is a bigger and better apfsds round on the lines of dm-53, a good heat round(packed with cl-20),clgm,a good aps and tusk upgrades.For arjun mk3 if army finally decides to go for a smoothbore then we can move for a higher caliber than 120mm say 140mm/152mm/155mm since russian are already placing a 152mm smoothbore on armata mbt and nato and chinese will follow.Also for arjun mk3 we need a better indegeneous 1500hp/1800hp engine.
Sorry, but if you will stick with rifled gun, you loose two important opportunities, first is capability to test and purchase more types of ammunition. Second you loose on foreing markets because world wide, rifled guns are not used anymore, especially in 120mm calliber.
As for HESH, HESH is also considered as obsolete type of ammunition world wide. I explained so many times why due to it's working mechanism, it is very simple to make HESH ineffective even against structures like bunkers. It is also cheap to make so.
As for "Armata" MBT, it will use new type of 125mm smoothbore gun the 2A82, but designers will also provide capability to rearm for 152mm 2A83.
NATO have several developed and tested 140mm guns, the US XM291, German NKPz-140 and also their French, British and Swiss analogs. So it is possible to start manufacturing of them, but a bit longer would take to actually design new ammunition for them. Also allready existing MBT's would need adaptation for these guns, as ar as I know only Swiss gun and US XM291 were ok to be installed in existing turrets, however at leas Americans and Germans designed new turrets for M1 and Leopard 2 to better fit 140mm in to them, I also know a French concept drawing of improved Leclerc with 140mm.
